Rewrite channels to be internally upgrade-able

The user-facing API-level change of this commit is that `SharedChan` is gone and `Chan` now has `clone`. The major parts of this patch are the internals which have changed.

Channels are now internally upgraded from oneshots to streams to shared channels depending on the use case. I've noticed a 3x improvement in the oneshot case and very little slowdown (if any) in the stream/shared case.

This patch is mostly a reorganization of the `std::comm` module, and the large increase in code is from either dispatching to one of 3 impls or the duplication between the stream/shared impl (because they're not entirely separate).

The `comm` module is now divided into `oneshot`, `stream`, `shared`, and `select` modules. Each module contains the implementation for that flavor of channel (or the select implementation for select).

Some notable parts of this patch

* Upgrades are done through a semi-ad-hoc scheme for oneshots and messages for streams
* Upgrades are processed ASAP and have some interesting interactions with select
* send_deferred is gone because I expect the mutex to land before this
* Some of stream/shared is straight-up duplicated, but I like having the distinction between the two modules
* Select got a little worse, but it's still "basically limping along"
* This lumps in the patch of deallocating the queue backlog on packet drop
* I'll rebase this on top of the "more errors from try_recv" patch once it lands (all the infrastructure is here already)

All in all, this shouldn't be merged until the new mutexes are merged (because send_deferred wasn't implemented).